Nylons are well known for their toughness, abrasion resistance, lubricity, and chemical resistance. Any number of polymeric materials may be coated with nylon polymers to enhance their properties accordingly. For example, sewing threads are typically coated with nylon (and lubricants in most cases) to protect them from abrasion during sewing operations. Furthermore, in thread bonding applications such coatings are applied to twisted multi-filament sewing threads in order to prevent fraying and unraveling (untwisting) of the individual filaments. See generally, a December 1990 brochure entitled “Elvamide® Product and Properties Guide”, describing their nylon multipolymer resins and available from E.I. DuPont de Nemours & Co., Wilmington, Delaware and Kohan, M. I., “Nylon Plastics Handbook” Hansen/Gardner Publications, Inc. (1995) pages 283-290.
Presently, nylon multi-polymers that are soluble in alcohols are available for coatings applications (see the reference to “Nylon Plastics Handbook” noted above). These types of nylons are sold commercially for a variety of coatings and adhesive applications. Generally these are readily soluble in organic solvents and are generally applied as alcoholic solutions. See for example brochures entitled “Elvamide® Nylon Multipolymer Resins, Properties and Uses” (September 1977), “Elvamide® & Nylon Multipolymer Resins for Thread Bonding” (October 1977), and “Elvamide® Product and Properties Guide” (December 1990) all from E.I. DuPont de Nemours and Company. However, there is an interest in aqueous-based nylon systems for these applications that eliminate or alleviate organic emissions.
To be effective, any viable water-soluble nylon coating system would require a readily available water-soluble nylon and a process for rendering the coating of this material water-insoluble either during or after the coating process. The insolubilization process is important for a number of end-use applications where subsequent exposure to water would occur.
Water-soluble polymers are either natural (biopolymers) or man-made. To be water-soluble these polymers are highly polar in nature. A large number of functional groups are used to impart polarity and, in turn, water-solubility to various polymers. Functional groups such as amines, hydroxyl, sulfonic acids, and carboxylic acids and their salts are commonly used to solubilize polymers. However, it is not uncommon for the reactive functional groups such as acids and amines to react with other acids and bases that they come in contact with during end-use, and this leads to changes in properties such as solubility. For an extensive review of this subject see “Water-Soluble Polymers”, Encyclopedia of Polymer Science & Engineering, Volume 17, pages 730-784, Second Edition, John Wiley & Sons (1989).
The patent literature includes various teachings regarding water-soluble nylons. U.S. Pat. No. 4,895,660 describes water-soluble sulfonated aromatic polyamides and polyureas that are cross-linked ionically with multi-valent metals for membranes, coatings, and adhesives. Japanese Patent Application 56-93704 describes a photosensitive composition comprising a water-soluble polyamide containing sodium sulfonate groups, a polymerizable unsaturated compound, and a sensitizer for printing plate applications. Japanese Patent Application 98007903A claims the use of alcohol-water solutions of nylons commonly known as PA66, PA46, PA6, and PA12 with a water-soluble methoxymethylated nylon, and thiocyanate salts in alcohol-water solvent as wiper blade coatings to improve performance and durability of the blades. U.S. Pat. Nos. 4,323,639 and 5,688,632 are both directed to water-soluble copolyamides containing polyether segments of 150 to 1500 molecular weight. These polyether-amide segments are obtained from polyether diamines and an aliphatic dicarboxylic acid. These water-soluble polyamides are used in conjunction with photopolymerizable compound and a photoinitiator for printing plate applications. The Elvamide® (October 1977) brochure and the “Nylon Plastics Handbook” mentioned above describe the ability of thermosetting resins such as epoxy, phenol-formaldehyde, and melamine-formaldehyde to cross-link nylon multi-polymers and improve the adhesion of the coating. The nylon multi-polymer reacts with these thermosetting resins to form thermoset-thermoplastic compositions. U.S. Pat. No. 4,992,515 describes the use of Cymel® 1135 available from Cytek Industries, Inc., a fully alkylated melamine-formaldehyde resin, and strong acid catalyst to crosslink nylon 6/66/69, nylon 6/66/610, and nylon 6/66/612 terpolymers applied from alcohol solutions. Other grades of Cymel® amino formaldehyde crosslinking agents are also available from Cytek.
The need for reduction or elimination of volatile organic compounds has been an objective in industry for a number of years. However, up to now there has been no report of a water-soluble nylon used as coatings and which is subsequently crosslinked with a water-soluble melamine-formaldehyde crosslinking agent to render the coating insoluble in water.
It is an object of the present invention to provide a water-soluble nylon that can be readily applied as a coating to substrates. It is a further object of the present invention to provide such nylon materials that can be rendered insoluble to water after their application. It is another object of the present invention to provide a polyamide coating composition that can be applied using a water solution. This water-soluble nylon is then crosslinked either during the coating process or subsequently thereafter rendering the water-soluble nylon insoluble in water. A feature of the present invention is its utility as a coating useful in a variety of applications including films, tubings, and shaped articles, and of particular interest sewing threads and filaments. Another feature of the present invention is its configuration as an aqueous-based system, thereby reducing organic emissions. An advantage of the present invention is its adaptability as a coating suitable for both nylon and polyester substrates. These and other objects, features and advantages of the present invention will become better understood upon having reference to the description of the invention herein.